This invention relates to a pulse doppler radar system for use on a target tracking missile in a situation where there might be two or more missiles having basically similar radar tracking systems, the missiles being mutually `friendly` and tracking the same target or closely spaced different targets. In such circumstances there is a danger that each radar's transmission will be received by the other, either direct or indirect (e.g. after reflection from the ground) and considered as a target. The deceived radar may then lock on to the `victim` radar and track it to destruction, or at least waste time in performing checks to eliminate it from consideration. This situation may come about despite significant separation of the radar transmission (r.f.) frequencies as may be seen by reference to FIGS. 1, 2 and 3 of the accompanying drawings.
FIG. 1 shows a typical radar transmission signal, comprising pulses of r.f. having a pulse duration of several microseconds and a pulse repetition frequency of perhaps 100 kHz. Such a signal has a frequency spectrum as shown in FIG. 2 comprising a center frequency component at the radar microwave frequency and a series of pulse sidebands of decreasing amplitude extending away from the center frequency component at spacings equal to the pulse repetition frequency. The center frequencies of two missile radars may be spaced apart significantly, e.g. by tens of megahertz so that there is little or no chance of mutual interference, but, as show in FIG. 3, the sidebands of one radar (shown in broken lines) may fall sufficiently close to the center frequency of another to be received as a doppler shifted return of that other.